The present invention relates to a collimator control method and apparatus and an X-ray CT (computed tomography) apparatus, and more particularly to a method and apparatus for controlling a collimator for defining an impingement position of an X-ray beam on an X-ray detector, and an X-ray CT apparatus comprising such a collimator control apparatus.
An X-ray CT apparatus acquires signals of transmitted X-rays for a plurality of views with respect to an imaging object by an X-ray emitting/detecting apparatus, and produces a tomographic image of the object by an image producing apparatus based on the signals of transmitted X-rays.
The X-ray emitting apparatus forms a cone-shaped X-ray beam emitted from a focus of an X-ray tube into a fan-shaped X-ray beam by a collimator, and projects the fan-shaped X-ray beam toward an imaging space.
The X-ray detecting apparatus detects the X-rays passing through the imaging space by a multi-channel X-ray detector that comprises a multiplicity of X-ray detector elements arranged in an array along the extent of the fan of the X-ray beam. Such an X-ray emitting/detecting apparatus is rotated (or scans) around the object to acquire the signals of transmitted X-rays for the plurality of views.
One of several types of the multi-channel X-ray detector is a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped X-ray beam to simultaneously receive an X-ray beam by the plurality of rows of the detector element array. Since such an X-ray detector can obtain X-ray detected signals for a plurality of slices together in one scan, it is used as an X-ray detector for efficiently performing a multi-slice scan.
Such X-ray detectors include an X-ray detector element array that comprises two rows and obtains projection data for two slices together. In such an X-ray detector, two rows of the array are disposed adjacent to each other in parallel, and illuminated by an X-ray beam equally divided in the thickness direction of the X-ray beam. The thicknesses of the X-ray beam impinging upon the two rows of the array at an isocenter of the object determines the slice thicknesses of tomographic images.
The X-ray tube undergoes an X-ray focus shift caused by thermal expansion or the like due to a temperature rise during use, which shift appears as displacement in the thickness direction of the X-ray beam after passing through an aperture of the collimator. When the X-ray beam shifts in the thickness direction, the division of the thickness of the X-ray beam becomes unequal between the two rows of the array. Equality between the slice thicknesses of two tomographic images is therefore lost.
It is therefore an object of the present invention to provide a collimator control method and apparatus for keeping a constant X-ray impingement position on an X-ray detector, and an X-ray CT apparatus comprising such a collimator control apparatus.
(1) In one aspect for solving the aforementioned problem, the present invention is a collimator control method, comprising the steps of: forming X-rays emanating from a focus of an X-ray tube into a fan-shaped beam by a collimator, and projecting the fan-shaped beam onto a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; and controlling the collimator based on the detected error so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position.
(2) In another aspect for solving the aforementioned problem, the present invention is a collimator control apparatus, comprising: an X-ray tube for generating X-rays emanating from a focus; a collimator for forming the X-rays into a fan-shaped beam; a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; error detecting means for detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; and control means for controlling the collimator based on the detected error so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position.
(3) In still another aspect for solving the aforementioned problem, the present invention is an X-ray CT apparatus, comprising: an X-ray tube for generating X-rays emanating from a focus; a collimator for forming the X-rays into a fan-shaped beam; a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; an error detecting apparatus for detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; a control apparatus for controlling the collimator based on the detected error so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position; a signal acquiring apparatus for acquiring X-ray detected signals for a plurality of views with an X-ray emitting/detecting system including the X-ray tube, the collimator and the detector element array rotated around an axis that is in parallel with the thickness direction of the fan-shaped beam; and a tomographic image producing apparatus for producing tomographic images for slices crossed by the fan-shaped beam based on the X-ray detected signals.
In the invention of the aspects described in (1)-(3), since an error in an impingement position of an X-ray beam is detected in the direction of the side-by-side arrangement of the detector element rows on the detector element array, and the collimator is controlled based on the detected error so that the impingement position of the X-ray beam coincides with a predetermined impingement position, the X-ray impingement position can be kept constant on the detector element array.
In the invention of these aspects, it is preferred to detect the error based on a ratio of a difference between X-ray detected signals to a sum of those X-ray detected signals, which X-ray detected signals are detected by X-ray detector elements that are adjacent in the direction of the side-by-side arrangement of the detector element rows, because an error measurement can be obtained independent of the magnitude of the X-ray detected signals.
(4) In still another aspect for solving the aforementioned problem, the present invention is a collimator control method, comprising the steps of: forming X-rays emanating from a focus of an X-ray tube into a fan-shaped beam by a collimator, and projecting the fan-shaped beam onto a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; and controlling the collimator based on the detected error so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position, wherein no control is performed when the error falls within a first range, the control is performed with a first proportional gain when the error exceeds the first range and falls within a second range larger than the first range, and the control is performed with a second proportional gain larger than the first proportional gain when the error exceeds the second range.
(5) In still another aspect for solving the aforementioned problem, the present invention is a collimator control apparatus, comprising: an X-ray tube for generating X-rays emanating from a focus; a collimator for forming the X-rays into a fan-shaped beam; a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; error detecting means for detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; and control means for controlling the collimator based on the detected error so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position, wherein the control means performs no control when the error falls within a first range, performs the control with a first proportional gain when the error exceeds the first range and falls within a second range larger than the first range, and performs the control with a second proportional gain larger than the first proportional gain when the error exceeds the second range.
(6) In still another aspect for solving the aforementioned problem, the present invention is an X-ray CT apparatus, comprising: an X-ray tube for generating X-rays emanating from a focus; a collimator for forming the X-rays into a fan-shaped beam; a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; an error detecting apparatus for detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; a control apparatus for controlling the collimator based on the detected error so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position, wherein the control apparatus performs no control when the error falls within a first range, performs the control with a first proportional gain when the error exceeds the first range and falls within a second range larger than the first range, and performs the control with a second proportional gain larger than the first proportional gain when the error exceeds the second range; a signal acquiring apparatus for acquiring X-ray detected signals for a plurality of views with an X-ray emitting/detecting system including the X-ray tube, the collimator and the detector element array rotated around an axis that is in parallel with the thickness direction of the fan-shaped beam; and a tomographic image producing apparatus for producing tomographic images for slices crossed by the: fan-shaped beam based on the X-ray detected signals.
In the invention of the aspects described in (4)-(6), since, in detecting an error in an impingement position of an X-ray beam in the direction of the side-by-side arrangement of the detector element rows on the detector element array, and controlling the collimator based on the detected error so that the impingement position of the X-ray beam coincides with a predetermined impingement position, no control is performed when the error falls within a first range, the control is performed with a first proportional gain when the error exceeds the first range and falls within a second range larger than the first range, and the control is performed with a second proportional gain larger than the first proportional gain when the error exceeds the second range, the control to keep a constant X-ray impingement position on the detector element array can be carried out rapidly and stably.
In the invention of these aspects, it is preferred to detect the error based on a ratio of a difference between X-ray detected signals to a sum of those X-ray detected signals, which X-ray detected signals are detected by X-ray detector elements that are adjacent in the direction of the side-by-side arrangement of the detector element rows, because an error measurement can be obtained independent of the magnitude of the X-ray detected signals.
(7) In still another aspect for solving the aforementioned problem, the present invention is a collimator control method, comprising the steps of: forming X-rays emanating from a focus of an X-ray tube into a fan-shaped beam by a collimator, and projecting the fan-shaped beam onto a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; removing high frequency components in the detected error; and controlling the collimator based on the error after the removal of the high frequency components so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position.
(8) In still another aspect for solving the aforementioned problem, the present invention is a collimator control apparatus, comprising: an X-ray tube for generating X-rays emanating from a focus; a collimator for forming the X-rays into a fan-shaped beam; a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; error detecting means for detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; high frequency component removing means for removing high frequency components in the detected error; and control means for controlling the collimator based on the error after the removal of the high frequency components so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position.
(9) In still another aspect for solving the aforementioned problem, the present invention is an X-ray CT apparatus, comprising: an X-ray tube for generating X-rays emanating from a focus; a collimator for forming the X-rays into a fan-shaped beam; a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; an error detecting apparatus for detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; a high frequency component removing apparatus for removing high frequency components in the detected error; a control apparatus for controlling the collimator based on the error after the removal of the high frequency components so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position; a signal acquiring apparatus for acquiring X-ray detected signals for a plurality of views with an X-ray emitting/detecting system including the X-ray tube, the collimator and the detector element array rotated around an axis that is in parallel with the thickness direction of the fan-shaped beam; and a tomographic image producing apparatus for producing tomographic images for slices crossed by the fan-shaped beam based on the X-ray detected signals.
In the invention of the aspects described in (7)-(9), since an error in an impingement position of an X-ray beam is detected in the direction of the side-by-side arrangement of the detector element rows on the detector element array, and the collimator is controlled based on the error with its high frequency components removed so that the impingement position of the X-ray beam coincides with a predetermined impingement position, the X-ray impingement position can be kept constant on the detector element array without being affected by the high frequency components in the error.
In the invention of these aspects, it is preferred to detect the error based on a ratio of a difference between X-ray detected signals to a sum of the X-ray detected signals, which X-ray detected signals are detected by X-ray detector elements that are adjacent in the direction of the side-by-side arrangement of the detector element rows, because an error measurement can be obtained independent of the magnitude of the X-ray detected signals.
Moreover, the removal of high frequency components may be achieved either by averaging processing or by low-pass filtering.
(10) In still another aspect for solving the aforementioned problem, the present invention is a collimator control method, comprising the steps of: forming X-rays emanating from a focus of an X-ray tube into a fan-shaped beam by a collimator, and projecting the fan-shaped beam onto a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; removing high frequency components in the detected error; and controlling the collimator based on the error after the removal of the high frequency components so that the impingement position of the fan-shaped beam coincides with the predetermined- impingement position, wherein no control is performed when the error falls within a first range, the control is performed with a first proportional gain when the error exceeds the first range and falls within a second range larger than the first range, and the control is performed with a second proportional gain larger than the first proportional gain when the error exceeds the second range.
(11) In still another aspect for solving the aforementioned problem, the present invention is a collimator control apparatus, comprising: an X-ray tube for generating X-rays emanating from a focus; a collimator for forming the X-rays into a fan-shaped beam; a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; error detecting means for detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement of the detector element rows on the detector element array; high frequency component removing means for removing high frequency components in the detected error; and control means for controlling the collimator based on the error after the removal of the high frequency components so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position, wherein the control means performs no control when the error falls within a first range, performs the control with a first proportional gain when the error exceeds the first range and falls within a second range larger than the first range, and performs the control with a second proportional gain larger than the first proportional gain when the error exceeds the second range.
(12) In still another aspect for solving the aforementioned problem, the present invention is an X-ray CT apparatus, comprising: an X-ray tube for generating X-rays emanating from a focus; a collimator for forming the X-rays into a fan-shaped beam; a detector element array comprising a plurality of detector element rows arranged side by side in the thickness direction of the fan-shaped beam, each of which detector element rows comprises a plurality of X-ray detector elements disposed in line in the extent direction of the fan-shaped beam; an error detecting apparatus for detecting an error between an impingement position of the fan-shaped beam and a predetermined impingement position in the direction of the side-by-side arrangement, of the detector element rows on the detector element array; a high frequency component removing apparatus for removing high frequency components in the detected error; a control apparatus for controlling the collimator based on the error after the removal of the high frequency components so that the impingement position of the fan-shaped beam coincides with the predetermined impingement position, wherein the control means performs no control when the error falls within a first range, performs the control with a first proportional gain when the error exceeds the first range and falls within a second range larger than the first range, and performs the control with a second proportional gain larger than the first proportional gain when the error exceeds the second range; a signal acquiring apparatus for acquiring X-ray detected signals for a plurality of views with an X-ray emitting/detecting system including the X-ray tube, the collimator and the detector element array rotated around an axis that is in parallel with the thickness direction of the fan-shaped beam; and a tomographic image producing apparatus for producing tomographic images for slices crossed by the fan-shaped beam based on the X-ray detected signals.
In the invention of the aspects described in (10)-(12), since, in detecting an error in an impingement position of an X-ray beam in the direction of the side-by-side, arrangement of the detector element rows on the detector element array, and controlling the collimator based on the error with its high frequency components removed so that the impingement position of the X-ray beam coincides with a predetermined impingement position, no control is performed when the error falls within a first range, the control is performed with a first proportional gain when the error exceeds the first range and falls within a second range larger than the first range, and the control is performed with a second proportional gain larger than the first proportional gain when the error exceeds the second range, the control to keep a constant X-ray impingement position on the detector element array can be carried out rapidly and stably without being affected by the high frequency components in the error.
In the invention of these aspects, it is preferred to detect the error based on a ratio of a difference between X-ray detected signals to a sum of the X-ray detected signals, which X-ray detected signals are detected by X-ray detector elements that are adjacent in the direction of the side-by-side arrangement of the detector element rows, because error measurement can be obtained independent of the magnitude of the X-ray detected signals.
Moreover, the removal of high frequency components may be achieved either by averaging processing or by low-pass filtering.
As described above in detail, the present invention can provide a collimator control method and apparatus for keeping a constant X-ray impingement position on an X-ray detector, and an X-ray CT apparatus comprising such a collimator control apparatus.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.